FIG. 10 illustrates an example of a joint structure of a branch connector of this type (see JP-A-10-213044). This branch connector has the following connecting structure. A pressure-receiving bearing surface 11-3 opening to the outside is formed at the position of a branch hole 11-2. The branch hole 11-2 provided on a peripheral wall of a main pipe rail 11 in the form of a circular pipe communicates with an internal flow passage 11-1. A screw sleeve (pipe-shaped sleeve nipple) 13 is welded or brazed to an outer peripheral wall of the main pipe rail 11 in the vicinity of the pressure-receiving bearing surface 11-3. A pressing bearing surface 12-3 is formed by a connecting head 12-2 provided at the tip of a branch connector 12. The branch connector 12 has a flow path 12-1 for communicating with the flow passage 11-1. The pressing bearing surface 12-3 contacts and engages with the pressure-receiving bearing surface 11-3. A tightening cap nut 14 is externally attached to a sleeve washer 15 which has been fitted to the branch connector 12 with clearance in advance, and is brought into screw-engagement with the screw sleeve 13. In this structure, the branch connector 12 is tightly fitted by pressure applied to the neck of the connecting head 12-2 to be thereby connected.
FIG. 11 illustrates another example of the joint structure of the branch connector (see JP-A-2001-82664). In this example, the joint structure of the branch connector is basically similar to the joint structure shown in FIG. 10, having the following structure: a pressure-receiving bearing surface 21-3 opening to the outside is formed at the position of a branch hole 21-2 which is provided on a peripheral wall of a main pipe rail 21 to communicate with an internal flow passage 21-1; a screw sleeve (pipe-shaped sleeve nipple) 23 is welded or brazed to a peripheral surface of the pressure-receiving bearing surface 21-3; a pressing bearing surface 22-3 is formed by a connecting head 22-2 provided at the tip of a branch connector 22 which has a flow path 22-1 for communicating with the flow passage 21-1; the pressing bearing surface 22-3 contacts and engages with the pressure-receiving bearing surface 21-3; a tightening cap nut 24 is externally attached to a sleeve washer 25 which has been fitted to the branch connector 22 with clearance in advance; and the tightening cap nut 24 is brought into screw-engagement with the screw sleeve 23 such that the branch connector 22 can be connected. In this structure, the sleeve washer 25 is externally fixed to the branch connector 22 by caulking or other method to be combined therewith into one piece so that the branch connector 22 can be connected.
FIG. 12 illustrates still another example of the joint structure of the branch connector (Japanese Patent Application No. 2000-174009). In this joint structure of the branch connector, the sleeve washer is split into two washers, each of which is externally fitted to the branch connector in such a manner as to be laterally slidable away from each other along the contact surface. The washer of the sleeve washer disposed near the nut is floatable, and the other washer disposed near the main pipe rail is caulked to the branch connector. In this structure: a flow passage 31-1 is provided inside a main pipe rail 31 in the direction of the axial center; a branch hole 31-2 is formed on a peripheral wall of the main pipe rail 31 keeping a distance from another branch hole in the axial direction; a pressure-receiving bearing surface 31-3 is formed on a peripheral surface of the branch hole 31-2 to be open to the outside; a screw sleeve (pipe-shaped sleeve nipple) 33 which is concentric with the pressure-receiving bearing surface 31-3 is welded or brazed to a main pipe rail 31 in such a position as to surround the pressure-receiving bearing surface 31-3; a pressing bearing surface 32-3 is formed by a connecting head 32-2 provided at the tip of a branch connector 32 which has a flow path 32-1 for communicating with the flow passage 31-1; the pressing bearing surface 32-3 contacts and engages with the pressure-receiving bearing surface 31-3; a tightening cap nut 34 is externally attached to a floating-type sleeve washer 35 which has been split into two parts and externally fitted to the branch connector 32 in advance; and the tightening cap nut 34 is brought into screw-engagement with the screw sleeve 33 such that the branch connector 32 is tightly fitted by pressure applied to the neck of the connecting head 32-2 and is thereby connected.
In the above-described joint structures of the branch connectors, the seal surface is not only a tapered surface but also a spherical surface. The pressing surface of the tightening nut is not only a flat surface but also a spherical surface. In the case of the spherical surface, the pressure-receiving surface of the sleeve washer is a spherical surface similarly to the pressing surface of the tightening nut, and is concentric with the seal surface.
While the screw sleeves 13, 23 and 33 are welded or brazed in the above examples, it is obvious that those may be formed integrally with the main pipe rails 11, 21 and 31 by forging.
However, the above-described related-art joint structures of the branch connectors have the following drawbacks.